1. Field of the Invention
This invention relates to an electrophotographic endless belt, in particular, an intermediate transfer belt, and also relates to a process cartridge and an electrophotographic apparatus which have the intermediate transfer belt and an electrophotographic photosensitive member.
2. Related Background Art
Besides rigid-body, drum-shaped members, flexible endless-belt-shaped members (electrophotographic endless belts) are conventionally used in intermediate transfer belts, electrophotographic photosensitive members, transfer-transport members, fixing members, and so forth used in electrophotographic apparatus, such as copying machines and laser beam printers.
Usually, in an electrophotographic apparatus, an electrophotographic endless belt is put over, and supported on, at least two rollers disposed on its inner-periphery side and is rotatably driven under application of any desired tension when used.
However, because of slight errors or scattering in the diameter, deflection, rotating-shaft straightness and roller-to-roller parallelism of the rollers supporting the electrophotographic endless belt, it is inevitable that the electrophotographic endless belt meander from side to side during its rotating drive.
Such meandering of the electrophotographic endless belt from side to side makes the exposure position and the transfer position deviate to cause image misregistration. Also, in the case of a full-color electrophotographic apparatus, it makes the position of image formation deviate for each color to cause color misregistration (or color shift) when color toner images are superimposed on the electrophotographic endless belt or on a transfer material transported on the electrophotographic endless belt.
Accordingly, in order to prevent the electrophotographic endless belt from meandering, various methods have ever been proposed. In recent years, methods in which a meandering-preventive member is provided on the inner periphery of a beltlike substrate of the electrophotographic endless belt to prevent the electrophotographic endless belt from meandering are proposed in a large number.
For example, a method is available in which a roller provided over the whole outer periphery thereof with a groove that may fit in the cross-sectional shape of such a meandering-preventive member is used and an electrophotographic endless belt provided with the meandering-preventive member over the whole inner periphery is rotated, making the meandering-preventive member fit in this groove of the roller to prevent the belt from meandering.
As another example, a method is available in which a roller having substantially the same length as the distance between the inner sides of meandering-preventive members provided on both ends of a beltlike substrate of an electrophotographic endless belt is used and the belt is put over this roller and is rotated making its both-end meandering-preventive members and the roller fit in each other to prevent the belt from meandering.
As still another example, a method is available in which a roller, provided on one end in the axial direction thereof with a terraced portion in which a meandering-preventive member of an electrophotographic endless belt fits, is used to prevent the electrophotographic endless belt from meandering.
The above methods can make the electrophotographic endless belt travel smoothly without meandering. This enables good images free of any image misregistration or color misregistration to be formed.
Meanwhile, usually, where the electrophotographic endless belt is used in an electrophotographic apparatus, it has some means for controlling the position at which a toner image begins to be written.
For example, Japanese Patent Application Laid open No. 9-96943, and so forth, discloses a method in which a mark (a position detection member) is provided on a beltlike substrate of an electrophotographic endless belt and the writing of an image is started upon detection of this mark. This method is preferable because the detection can very inexpensively be made and also the apparatus can be made compact.
Now, usually, electrophotographic endless belts mostly have a small layer thickness from the viewpoint of making electrophotographic apparatus compact and light-weight, and are also required to have a certain amount of flexibility because they are used in the state in which the belt is put over rollers having a small diameter.
On the other hand, the meandering-preventive member fitted to a beltlike substrate of the electrophotographic endless belt is required to have a rigidity high enough to be durable with respect to the draw force of the electrophotographic endless belt.
Where the beltlike substrate of such a thin-film and flexible electrophotographic endless belt is provided with a rigid meandering-preventive member, a slight difference is produced in the flexing degree of the electrophotographic endless belt when the electrophotographic endless belt is put over the rollers, because there is a difference in stiffness (nerve or rigidity) between the part provided with the meandering-preventive member and the part not provided with it.
In the case when the meandering-preventive member is provided on the inner periphery of the beltlike substrate of the electrophotographic endless belt and the position detection member is provided on the outer periphery of that part, it has occurred in conventional cases that, as shown in FIG. 7, a meandering-preventive member 82 fitted in a groove 86 of a roller 87 rises because of this slight difference in flexing properties and consequently a beltlike substrate 81 of an electrophotographic endless belt and a position detection member 83 also rise to make any accurate detection impossible to cause image misregistration (reference numeral 84 denotes a light-projecting part of a position detection sensor, and reference numeral 85 denotes a light-receiving part of the position detection sensor).
It is also the case for the meandering-preventive member that, usually, a member cut beforehand to a length adjusted to the inner-peripheral length of the beltlike substrate is attached to the inner periphery of the beltlike substrate. In such a case, it is unavoidable for the meandering-preventive member to have a joint. In particular, where the position detection member is present on the joint, it is impossible to make any accurate position detection because of an extreme difference in flexing properties. In order to avoid this, the joint of the meandering-preventive member may be avoided when the position detection member is fitted, or the position of the position detection member may be avoided when the meandering-preventive member is fitted. However, taking account of a mass production process, the addition of a step of determining and avoiding the joint of the meandering-preventive member or the position of the position detection member causes a lowering of productivity or an increase in management, resulting in an increase in cost.
Accordingly, as a means for preventing the meandering-preventive member from rising, a method is available in which the tension (belt tension) applied when the electrophotographic endless belt is put over is made higher. There, however, is a possibility that making the tension higher causes a creep of the electrophotographic endless belt to shorten its lifetime. Also, too a high belt tension may further promote the meandering of the electrophotographic endless belt.
Conventionally, in order to solve such problems, it has been necessary to use a meandering-preventive member having a relatively low rigidity. However, the use of such a meandering-preventive member having a low rigidity may weaken the effect of preventing the belt from meandering in the width direction. In a bad case, it has even occurred that the meandering-preventive member runs on the roller.
In particular, where a process cartridge, in which an electrophotographic photosensitive member and an intermediate transfer belt are integrally supported, is used, differently from a case in which it is actually installed and used in the main body of an electrophotographic apparatus, it may often undergo many vibrations or be placed in a high-temperature and high-humidity environment for a long time during distribution in the market. When it is placed in such a severe environment for a long time, the progress of the creep of the belt is accelerated, and moreover the belt may come to have the habit of bending (or permanent bending) as a result of compression set. When the position detection member is present here, a problem may arise such that any accurate position detection can not be made. For such reasons, the above problems may more remarkably arise when the process cartridge in which an electrophotographic photosensitive member and an intermediate transfer belt are integrally supported is used.